Tessellation Shader vs Geometry Shader

Tessellation shader는 재귀 세분할이다.
반면 geometry shader는 primitive 단위로 적용된다.

The tessellation shader is for recursive subdivision. An important features is adjacency information so you can do smoothing correctly and not wind up with gaps. You could do some limited subdivision with a geometry shader, but that's not really what its for.

Geometry shaders operate per-primitive. For example, if you need to do stuff for each triangle (such as this), do it in a geometry shader. I've heard of shadow volume extrusion being done. There's also "conservative rasterization" where you might extend triangle borders so every intersected pixel gets a fragment. Examples are pretty application specific.